[docs] pre-hook for apidocs

[docs] pre-hook for apidocs

This patch adds pre-hook script which is newly supported by opnfvdocs
and can be used to generate apidocs in yardstick.

This patch also updates .gitignore ;

  - remove '/releng/' as it isn't needed any more

  - add '/docs/apidocs/yardstick*.rst' as those files are generated
    by sphinx and aren't needed to be stored in the repository

JIRA: RELENG-133

Change-Id: I7812b2d6a69b9feb37b6c592c978f91b335c0a1c
Signed-off-by: Ryota MIBU <r-mibu@cq.jp.nec.com>